1. Field of the Invention
The present invention relates generally to a method for performing handovers in mobile telecommunication systems, and more particularly, to a method for performing inter system handovers using different telecommunication methods.
2. Description of the Related Art
GSM (Global System for Mobile communication) is a digital mobile phone system favored in the vast majority of nations in Europe and other areas. The GSM is a variation of TDMA (Time Division Multiple Access), and in fact, it is one of three major digital radio (cordless) phone technologies together with TDMA and CDMA (Code Division Multiple Access), which are currently being used the most. The GSM digitizes and compresses data, and sends the data plus data of another two users to one channel. Here, each data is sent to its own specific time band. Such GSM can be carried out in both 900 MHz and 1800 MHz. GPRS (General Packet Radio Services) is a newly developed packet-based mobile telecommunication services based on the GSM. The GPRS guarantee sustainable data service at the data speed of 56 Kbps to 114 Kbps to mobile station (MS) users. The GPRS is an evolutionary step heading for UMTS (Universal Mobile Telecommunication Service), a third generation mobile telephony system, for providing high-speed, high-quality voice and multimedia services.
Although the UMTS is based on GSM communication standard, it uses wideband CDMA technologies as well. Therefore, the UMTS can provide uniform services, i.e. transmitting packet-based text, digitized voice or video, and multimedia data at a higher data speed than 2 Mbps (mega bit per second) to any cellular phone or computer user no matter which part of the world they are located in. The UMTS is built on the virtual connection, that is, the packet switching system connection using a packet protocol, such as, IP (Internet Protocol), whereby it is connectable to any type of end within the network.
Such UMTS is called a third generation mobile telephone system, which is currently under development as a successor to the GSM/GPRS and other existing digital systems. A problem still arises though because technical transition from the second generation cellular system (i.e. GSM/GPRS) to the third generation mobile telephony system (i.e. UMTS) will take place step by step, probably years, many network operators feel the necessity of providing users with mobile terminals that work in both GSM/GPRS and UMTS modes for the time being. As such, a dual mode user equipment or dual mode mobile terminal (it is interchangeably used with “user equipment”) that supports UMTS as well as GSM/GPRS was developed. Further, since many mobile terminal users travel across the nations for business, moving from one region to another region where different radio access technologies are covered via certain nations with certain types of roaming services, there is a necessity for a multi-mode mobile terminal. Therefore, to develop the third generation mobile telephone system, it is absolutely important to develop a dual mode mobile terminal. In this way, even though UMTS is not yet completely developed, mobile subscribers can be provided with multi services by using dual mode mobile terminals that supports both UMTS and GSM/GPRS whenever necessary. Consequently, when they move to a region where there is no coverage provided for UMTS users, the call still can be set up on an existing network, e.g. GSM 900 or GSM 1800.
As discussed before, when a dual mode mobile terminal in a call mode in a region where the GSM/GPRS or UMTS provides coverage moves to another region where another radio access technology (RAT) provides coverage, it should be possible to perform a handover, switching channel to a concerned RAT network while maintaining the existing GSM/GPRS or UMTS connection. However, currently provided technologies are not equipped with this handover capability. Hence, when mobile terminals move between regions where different networks provide coverage, their calls are disconnected, thereby causing much inconvenience to users.